Vacuum interrupter

ABSTRACT

A supporting member for the tubular intermediate metallic shield of a vacuum interrupter, which comprises a first metallic strip secured to the outer surface of the shield, second and third metallic strips formed integrally with the first strip and each extending from the respective sides of the first strip at a predetermined angle, one of the side edges of each of the second and third metallic strips engaging with an inclined side of a trapezoidal projection formed along the internal surface of dielectric cylindrical casing which forms the vacuum envelope, the second and third strips being bent so that the folding lines between the main plane of the second and third strips and the main plane of the first strip are generally parallel to the axis of the tubular shield.

United States Patent Inventors Oslmn Saito;

Tahmasa Saito; Yutakn lwashlrna, all of Hitachi-ski, Japan 1 :1

Jan. 26, 1970 June 22, 1971 Hitachi, Ltd. Tnkym, Japan Jan. 24, 1969 Japan Appl. No. Filed Patented Assignee Priority VACUUM INTERRUFIER 10 Claims, 4 Drawing Figs.

[LS-Cl.

Int. Cl.

Boll! 33/66 Field Search 2001144 B,

References Cited UNITED STATES PATENTS 6/1959 Crouch 2,980,782 4/1961 Schneider 200/144 B 3,048,681 8/1962 Polinko, Jr. 200/144 B 3,048,682 8/1962 Yeras 200/144 B FOREIGN PATENTS l,4l 1,483 8/1965 France 200/144 B Primary Examiner-Robert S. Macon Attorney-Craig, Antonelli, Stewart & Hill ABSTRACT: A supporting member for the tubular intermediate metallic shield of a vacuum interrupter, which comprises a first metallic strip secured to the outer surface of the shield, second and third metallic strips formed integrally with the first strip and each extending from the respective sides of the first strip at a predetermined angle, one of the side edges of each of the second and third metallic strips engaging with an inclined side of a trapezoidal projection formed along the internal surface of dielectric cylindrical casing which forms the vacuum envelope, the second and third strips being bent so that the folding lines between the main plane of the second and third strips and the main plane of the first strip are generally parallel to the axis of the tubular shield.

VACUUM INTERRUPTER BACKGROUND OF THE IN Vl'lN'lION This invention relates to a vacuum interrupter and more particularly relates to a supporting member for the tubular intermediate metallic shield of a vacuum interrupter.

A vacuum interrupter comprises a highly evacuated envelope consisting generally of a dielectric cylindrical casing, e.g. made of glass or ceramics, with a pair of metallic end caps closing off the ends of the casing, a pair of separable contacts or electrodes disposed within the envelope, and a tubular metallic shield disposed between the casing and the electrodes and encircling the electrodes to prevent vapors dispersed from the contacts upon contact separation from producing condensation of arc-generated metallic particles on the internal surface of the casing.

This shield is called an intermediate shield by reason of the fact that the shield is isolated from both electrodes so as to improve the electric field distribution in the envelope.

Supporting members for the intermediate shield of a vacuum interrupter which have been proposed prior to this invention are as follows:

One type of supporting member is constructed so as that the intermediate shield is supported by engaging metal fittings, which form one part of the supporting member, secured integrally in the dielectric casing during the production thereof, with another part of the supporting member being secured to the outer surface of the intermediate shield. However, this known supporting member has the disadvantage that it reduces the mechanical strength of the casing and causes air ereepage into the envelope to decrease the degree of vacuum of it. Further, in case the casing is made of a ceramic or similar material, it is very difficult to secure the metal fittings integrally in the ceramic body during the production thereof.

Another known supporting member is constructed so as that the intermediate shield is supported by engaging metal fittings, which form one part of the supporting member, adhered to the internal surface of the casing after the production thereof by metallizing the surface thereof, with another part of the supporting member being secured to the outer surface of the intermediate shield. This supporting member has the disadvantage that it requires so much time for metallizing the internal surface of the casing and adhering the metal fittings to the portion thereof that it is not applicable to mass production techniques by which the vacuum interrupter may be economically manufactured. Further, adhesion between the metal fittings and the internal surface of the casing is generally insufficient because of the differences in the coefficient of thermal expansion between the metal fittings and the casing.

Still another known supporting member is constructed so that the intermediate shield is supported by engaging rectangular metallic plates, which are secured at their center part around the predetermined position of the outer surface of the intennediate shield by means of spot welding and so forth, with the inclined plane of a trapezoidal projection fon'ned along the internal surface of the casing, by bending both parts of the rectangular plate divided by the welded portion so as to position the main plane of the plate in contact with the inclined plane of the trapezoidal projection and to hold the projection therebetween. That is, rectangular plates are bent so that the folding lines between the main plane of the plate and the plane of the intermediate shield are perpendicular to the axis of the tubular intermediate shield.

Although the time required for fixing this third supporting member to the intermediate shield and the casing in extremely short as compared to the above-mentioned supporting member; the ability of this element to provide dependable support is less than satisfactory. After the structural elements of the vacuum interrupter are assembled, it is necessary to heat up the vacuum interrupter in a furnace at a temperature between 400 C. and 500 C. to free the interrupter of gases absorbed internally of the contact body and other materials in the envelope so as to preclude evolution of these gases during current interruption. Accordingly, the tenacity of the rectangular metallic plates decreases and the brittleness of them increases due to this heat treatment process. The result is that the engagement between the rectangular metallic plates and the trapezoidal projection of the casing becomes incomplete after the heating up operation.

Therefore an object of the present invention is directed to the improvement of the above-explained third supporting member for the intermediate shield of a vacuum interrupter.

One object of the invention is to provide a vacuum interrupter having a supporting member which firmly fixes the tubular intermediate shield to the casing of the envelope.

Another object of the invention is to provide a vacuum interrupter having a supporting member which can be fixed to the tubular intermediate shield and the casing in an extremely reduced time.

SUMMARY OF THE INVENTION In carrying out the present invention in one form, there is provided a vacuum interrupter which comprises a highly evacuated envelope consisting of a dielectric cylindrical casing and a pair of metallic end caps closing off the ends of the casing; a pair of separable electrodes disposed within the envelope; a tubular metallic shield disposed between the casing and the electrodes and encircling the electrodes; a trapezoidal projection formed along the circumference of the internal surface of the casing substantially at its center portion; a supporting member secured to the outer surface of the tubular metallic shield and engaging with the trapezoid projection for supporting the tubular metallic shield, said supporting members having at least two first metallic strips secured to the outer surface of the tubular metallic shield,,at least one second metallic strip formed integrally with each of the first metallic strips and extending from one side of the first metallic strip and at least one third metallic strip formed integrally with each of the first metallic strips and extending from the other side of the first metallic strip, said second and third metallic strips each consisting of a first portion extending immediately from the first metallic strip and a second portion extending from the first portion, each second and third metallic strip being bent so that the folding or bending lines between the main plane of the second and third metallic strips and the main plane of the first metallic strip are generally parallel to the axis of the tubular metallic shield, the directions of the bending of the second and third strips being opposite to each other. One side plane of the each first portion engages with a respective inclined surface of the trapezoid projection, one side edge of each second portion contacting the internal surface of the casing and another side edge of each second portion contacting the outer surface of the tubular metallic shield. In other words, each first portion of the second and third metallic strips of the supporting member of this invention engages with each inclined surface of the trapezoidal projection by a side edge thereof, not by the main plane thereof. The moment of inertia of each first portion becomes extremely larger than the above-explained third supporting member, accordingly the flexural rigidity of the supporting member is maintained at a high value, not withstanding the decrease of the tenacity of the supporting member and the increase of the brittleness of it after the heating up operation of the vacuum interrupter.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of our invention, reference may he had to the following description taken in conjunction with the accompanying drawings, wherein;

FIG. I is a cross-sectional view of a vacuum interrupter embodying one form of our invention.

FIG, 2 is a plane view of a part of a metal fitting which is not yet formed into a supporting member used in the interrupter of FIG. 1.

FIG. 3 is a plane view ofa part ofa supporting member into which the metal fitting shown in FIG. 2 has been fon'ned.

FIG. 4 illustrates a modified embodiment of metal fitting shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the interrupter of FIG. 1, there is shown a highly evacuated envelope 10 comprising a cylindrical casing 11 of suitable insulating material, such as a ceramic, and a pair of metallic end caps 12 and 13 closing off the end of the easing. Suitable seals 14 are provided between the end caps and the casing to render the envelope vacuum tight. The normal pressure within the envelope 10 under static condition is lower than l 4 mm. of mercury, so that a reasonable assurance is had that the mean free path for electrons will be longer than the potential breakdown paths in the envelope.

Located within the envelope is a pair of relatively movable disc-shaped contacts, or electrodes, 17 and I8 shown in their contacted or closed-circuit position. The upper contact is a stationary contact suitably secured to a conductive rod 17a, which at its upper end is united to the upper end cap 12. The lower contact 18 is movable contact joined to a conductive operating rod 180, which is suitably mounted for vertical movement. The operating rod 180 projects through an opening in the lower end cap 13. And a flexible metallic bellows [9 provides a seal about the rod 180 to allow for vertical movement of the rod without impairing the vacuum inside the envelope 10. As shown in FIG. I, the bellows I9 is secured in sealed relationship at its respective opposite ends to the operating rod 18a and the end cap 13.

Coupled to the lower end of the operating rod 18a, suitable actuating means (not shown) of any conventional configuration is provided for driving the movable contact 18 downwardly out of engagement with the stationary contact 17 so as to open the interrupter. The actuating means is also capable of returning the contact 18 to its illustrated position so as to close the interrupter.

The are that is established across the gap between the electrodes upon contact-separation vaporizes some of the contact material, and these vapors are dispersed from the arcing gap toward the envelope. In the illustrated interrupter, the internal insulating surfaces of the casing 11 are protected from the condensation of arc-generated metallic particles thereon by means of a tubular metallic shield supported on a trapezoid projection 11a formed along the internal surface of the casing through a supporting assembly 20 which will soon be explained in great detail. This shield acts to intercept and condense arc-generated metallic vapor before they can reach the internal surface of the casing 11. To prevent the vapor from condensing on the metallic bellows l9, shield 16 is provided on the operating rod 180 so as to encircle the metallic bellows 19.

Referring now to the FIG. 2, the metal fitting 20 is formed by stamping out a metallic plate made of a material which has less absorbed gases therein and/or a lower vapor pressure, e.g., stainless steel. The thickness of the metal fitting 20 is determined by the weight of the intermediate shield 15 which is supported on the trapezoidal projection 11a of the casing 11. Metal fitting 20 comprises a first metallic strip 21 and second and third metallic strips 22 and 23 formed integrally with the first strip and each extending from respective sides of the first metallic strip 21. Each of the second and third metallic strips 22 and 23 consist of a first portion 220 and 23a extending immediately from the first metallic strip 21 and a second portion 22b and 23b extending from each of the first portions 22a and 23a, respectively.

The first portion 22a of the second metallic strip 22 and the first portion 23a of the third metallic strip 23 extend with such an angle to the longitudinal direction of the first metallic strip 21 that one is directed at an acute angle and the other is directed at an obtuse angle, so that one of the side edges of each of the first portions 220 and 23a engages firmly with respective inclined surfaces of the trapezoidal projection when the second and third metallic strips are bent in the directions indicated by the arrows in FIG. 2. Similarly, the second portion 22b of the second metallic strip 22 and the second portion 23b of the third metallic strip 23 are formed so that one of the side edges of each of the second portions 22b and 23b contact firmly the internal surface of the casing II and the other of the edges contacts the outer surface of the intermediate tubular shield 15.

It is preferable that the first metallic strip be secured to the outer surface of the intermediate shield continuously along the whole circumferential outer surface of the tubular shield to make it easy to adjust the supporting member with respect to the trapezoidal projection Ila of the casing Il. Metal fittings having one second and one third metallic strip each extending from the respective sides of a first metallic strip or having two second and two third metallic strips each extending from the respective sides of a first metallic strip, as shown in FIG. 2, may be used as a unit of the supporting member.

FIG. 3 illustrates a plane view of a completed supporting member, the metal fittings being formed as shown in FIG. 2 in accordance with the above explanation. Both inclined surfaces of the trapezoidal projection lla are held tightly between one of the side edges 22c and 23c of each first portion 22a and 23a, and the intermediate shield can be firmly sup ported by the trapezoidal projection Ila of the casing II.

The moment of inertia of each one of the side edges 22c and 230 of each first portion 220 and 230 which engage with the inclined surfaces of the trapezoid projection 11a of the casing is extremely large, accordingly the fiexural rigidity of the supporting member is maintained at a high value, not withstanding the decrease in the tenacity of the supporting member and the increase of the brittleness of it after the heating up operation on the vacuum interrupter.

Each one of the side edges 22d and 23d of the second portion 22b and 23b contact the internal surface of the casing and each of the other side edges (not shown) of the second portion 22b and 2312 contact the outer surface of the intermediate shield.

FIG. 4 shows a modification of the metal fitting shown in FIG. 2 in that it is provided with additional means for supporting the intermediate shield 15 more firmly by the casing 11. Metal fitting 20 of FIG. 4 has abutting portions 24 extending from the one side of each of the first portions 22a and 23a and from the one side of each of the second portions 22!: and 23b that engage with the inclined surfaces of trapezoidal projection and the internal surface of the casing 11 when the second and third metallic strips 22 and 23 are bent in the directions indicated by the arrows in FIG. 4.

The abutting portions 24 are intended to increase the contact surface with the inclined surface of trapezoidal projection 11a and the internal surface of the casing II and to prevent the circumferential displacement of the intermediate shield 15. The abutting portions 24 are bent in such a manner that the planes of the abutting portion 24 become perpendicular to the plane of the second and third metallic strips 22 and 23, respectively, during the above-explained bending operation of each of the second and third metallic strips 22 and 23.

The fixing operation of the intermediate shield 15 to the casing 11 are as follows;

The first metallic strip 21 of the metal fitting 20 shown in FIG. 2 or FIG. 4 is secured along the circumferential outer surface of the intermediate shield 15 by means of spot welding, while adjusting the position of the first metallic strip 21 to fit to the trapezoidal projection lla. Next, after bending each of the second metallic strips by using a proper tool in the direction indicated by the arrows in FIG. 2 or FIG. 4 to the extent that the angle formed between the first metallic strip and the second metallic strip equals a predetermined value, e.g., 45, intermediate shield 15 is inserted into the casing 11 and the side planes 22c of each first portion 22a of the second metallic strips 22 engage with the upper inclined surface of the trapezoidal projection 11a. After that, bending each of the third metallic strips in the direction indicated by the arrows in FIG. 2 or FIG. 4 by using the tool which is inserted in the space between the casing 11 and the intermediate shield 15 from the bottom of the casing 11 so as to engage lightly with the lower inclined surface of the trapezoidal projection. And next, further bending each of the second metallic strips in the same direction indicated by the arrow using the tool which in turn is inserted from the top of the casing 11 so as to firmly engage with the upper inclined surface of the trapezoidal projection, and these operations are completed. Although it is preferable that the angle formed between the plane of the first metallic strip 21 and the second or the third metallic strip 22 or 23 is 90 after finishing the fixing operation so as to obtain the maximum moment of inertia of the supporting beams consistent with the second and the third metallic strips, it is not limited to that value. The adjustment of the angle is very useful to compensate for the production tolerance of the trapezoidal projection Ila and metal fittings 20. In addition, it is not necessary that the plane of the second metallic strip and the plane of the third metallic strip be coextensive after finishing the fixing operation. 1

Qlthough the present invention has been described with reference to but a single embodiment, it is to be understood that the scope of the invention is not limited to the specific details thereof, but is susceptible of numerous changes and modifications as would be apparent to one with normal skill in the pertinent technology.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. a vacuum interrupter comprising:

a. a highly evacuated envelope formed by a dielectric cylindrical casing and a pair of metallic end caps closing off the ends of said casing,

b. at least a pair of separable electrodes disposed within said envelope, a tubular metallic shield disposed between said casing and said electrodes encircling said electrodes,

d. a trapezoidal projection formed along the circumference of the internal surface of said casing substantially at its center portion,

e. a supporting member secured to the outer surface of said tubular metallic shield and engaging with said trapezoidal projection for supporting said tubular metallic shield, said supporting member having at least tow first metallic strips secured to the outer surface of said tubular metallic shield, at least one second metallic strip formed integrally with each of said first metallic strips and extending from one side of said first metallic strip and at least one third metallic strip formed integrally with each of said first metallic strips and extending from the other side of said first metallic strip, each of said second and third metallic strips consisting of first portion extending immediately from said first metallic strip and a second portion extending from said first portion, each of said second and third metallic strips being bent so that the folding lines between the main plane of said second and third metallic strips and the main plane of said first metallic strip are generally parallel to the axis of said tubular metallic shield, the direction of the bending of said second and third metallic strips being opposed to each other, one side plane of each of said first portions engaging with the respective inclined surface of said trapezoidal projection, one side edge of each of said second portions contacting the internal surface of said casing and the other side edge of each of said second portions contacting the outer surface of said tubular metallic shield.

2. The vacuum interrupter of claim 1, in which each of said first and second portions have an abutting portion extending from one side of each of said first and second portions, each of said abutting portions being bent so that each plane of said abutting portion becomes generally perpendicular to the plane of said first and second portions, and each of said abutting portions engages with either the inclined plane of said trapezoidal projection or the internal surface of said casing.

3. The vacuum interrupter ofclaim 1. in which n..- fim nnrangle to said first strip and said second portion of each of said second and third strips is disposed along a line substantially transverse to said first strip.

4. The vacuum interrupter of claim 3, in which the first portion of said second strip extending in an opposite direction from said first strip compared to the first portion of said third strip.

5. The vacuum interrupter of claim 4, in which said supporting member includes a pair of second metallic strips extending from one side of said first strip and a pair of third metallic strips extending from the other side of said first metallic strips, the first portions of each of said second strips being directed toward each other and the first portions of each of said third strips being directed away from each other.

6. A vacuum interrupter comprising:

a. a highly evacuated envelope formed by a dielectric cylindrical casing and a pair of metallic end caps closing off the ends of said casing,

b. at least a pair of separable electrodes disposed within said envelope,

c. a tubular metallic shield disposed between said casing and said electrodes and encircling said electrodes,

d. a trapezoidal projection formed along the circumference of the internal surface of said casing substantially at its center portion,

e. a supporting member secured to the outer surface of said tubular metallic shield and engaging with said trapezoidal projection for supporting said tubular metallic shield, said supporting member having a first metallic strip secured to the outer surface of said tubular metallic shield, at least two second metallic strips formed integrally with said first metallic strip and extending from one side of said first metallic strip and at least tow third metallic strips formed integrally with said first metallic strip and extending from the other side of said first metallic strip, each of said second and thirdmetallic strips consisting of a first portion extending immediately from said first metallic strip and a second portion extending from said first portion, each of said second and third metallic strips being bent so that the folding lines between the main plane of said second and third metallic strips and the main plane of said first metallic strip are generally parallel to the axis of said tubular metallic shield, the directions of the bending of said second and third metallic strips being opposed to each other, one side edge of each of said first portions engaging with the respective inclined surface of said trapezoidal projection, the one side edge of each of said second portions contacting the internal surface of said casing and the other side edge of each of said second portions eontacting the outer surface of said tubular metallic shield.

7. The vacuum interrupter of claim 6, in which each of said first and second portions have an abutting portion extending from one side of each of said first and second portions, each of said abutting portions being bent so that the plane of each of said abutting portions becomes generally perpendicular to the plane of said first and second portions, and each of said abutting portions engages with either the inclined surfaces of said trapezoidal projection or the internal surface of said casmg.

8. The vacuum interrupter of claim 6, in which the first portion of each of said second and third strips is disposed at an angle to said first strip and said second portion of each of said second and third strips is disposed along a line substantially transverse to said first strip.

9. The vacuum interrupter of claim 8, in which the first portion of said second strip extending in an opposite direction from said first strip compared to the first portion of said third strip.

10. The vacuum interrupter of claim 9, in which said supporting member includes a pair of second metallic strips extending from one side of said first strip and a pair of third metallic strips extending from the other side of said first metallic strip, the first portions of each of said second strips being 

1. A VACUUM INTERRUPTER COMPRISING: A. A HIGHLY EVACUATED ENVELOPE FORMED BY A DIELECTRIC CYLINDRICAL CASING AND A PAIR OF METALLIC END CAPS CLOSING OFF THE ENDS OF SAID CASING, B. AT LEAST A PAIR OF SEPARABLE ELECTRODES DISPOSED WITHIN SAID ENVELOPE, C. A TUBULAR METALLIC SHIELD DISPOSED BETWEEN SAID CASING AND SAID ELECTRODES ENCIRCLING SAID ELECTRODES, D. A TRAPEZOIDAL PROJECTION FORMED ALONG THE CIRCUMFERENCE OF THE INTERNAL SURFACE OF SAID CASING SUBSTANTIALLY AT ITS CENTER PORTION, E. A SUPPORTING MEMBER SECURED TO THE OUTER SURFACE OF SAID TUBULAR METALLIC SHIELD AND ENGAGING WITH SAID TRAPEZOIDAL PROJECTION FOR SUPPORTING SAID TUBULAR METALLIC SHIELD, SAID SUPPORTING MEMBER HAVING AT LEAST TOW FIRST METALLIC STRIPS SECURED TO THE OUTER SURFACE OF SAID TUBULAR METALLIC SHIELD, AT LEAST ONE SECOND METALLIC STRIP FORMED INTEGRALLY WITH EACH OF SAID FIRST METALLIC STRIPS AND EXTENDING FROM ONE SIDE OF SAID FIRST METALLIC STRIP AND AT LEAST ONE THIRD METALLIC STRIP FORMED INTEGRALLY WITH EACH OF SAID FIRST METALLIC STRIPS AND EXTENDING FROM THE OTHER SIDE OF SAID FIRST METALLIC STRIP, EACH OF SAID SECOND AND THIRD METALLIC STRIPS CONSISTING OF FIRST PORTION EXTENDING IMMEDIATELY FROM SAID FIRST METALLIC STRIP AND A SECOND PORTION EXTENDING FROM SAID FIRST PORTION, EACH OF SAID SECOND AND THIRD METALLIC STRIPS BEING BENT SO THAT THE FOLDING LINES BETWEEN THE MAIN PLANE OF SAID SECOND AND THIRD METALLIC STRIPS AND THE MAIN PLANE OF SAID FIRST METALLIC STRIP ARE GENERALLY PARALLEL TO THE AXIS OF SAID TUBULAR METALLIC SHIELD, THE DIRECTION OF THE BENDING OF SAID SECOND AND THIRD METALLIC STRIPS BEING OPPOSED TO EACH OTHER, ONE SIDE PLANE OF EACH OF SAID FIRST PORTIONS ENGAGING WITH THE RESPECTIVE INCLINED SURFACE OF SAID TRAPEZOIDAL PROJECTION, ONE SIDE EDGE OF EACH OF SAID SECOND PORTIONS CONTACTING THE INTERNAL SURFACE OF SAID CASING AND THE OTHER SIDE EDGE OF EACH OF SAID SECOND PORTIONS CONTACTING THE OUTER SURFACE OF SAID TUBULAR METALLIC SHIELD.
 2. The vacuum interrupter of claim 1, in which each of said first and second portions have an abutting portion extending from one side of each of said first and second portions, each of said abutting portions being bent so that each plane of said abutting portion becomes generally perpendicular to the plane of said first and second portions, and each of said abutting portions engages with either the inclined plane of said trapezoidal projection or the internal surface of said casing.
 3. The vacuum interrupter of claim 1, in which the first portion of each of said second and third strips is disposed at an angle to said first strip and said second portion of each of said second and third strips is disposed along a line substantially transverse to said first strip.
 4. The vacuum interrupter of claim 3, in which the first portion of said second strip extending in an opposite direction from said first strip compared to the first portion of said third strip.
 5. The vacuum interrupter of claim 4, in which said supporting member includes a pair of second metallic strips extending from one side of said first strip and a pair of third metallic strips extending from the other side of said first metallic strips, the first portions of each of said second strips being directed toward each other and the first portions of each of said third strips being directed away from each other.
 6. A vacuum interrupter comprising: a. a highly evacuated envelope formed by a dielectric cylindrical casing and a pair of metallic end caps closing off the ends of said casing, b. at least a pair of separable electrodes disposed within said envelope, c. a tubular metallic shield disposed between said casing and said electrodes and encircling said electrodes, d. a trapezoidal projection formed along the circumference of the internal surface of said casing substantially at its center portion, e. a supporting member secured to the outer surface of said tubular metallic shield and engaging wIth said trapezoidal projection for supporting said tubular metallic shield, said supporting member having a first metallic strip secured to the outer surface of said tubular metallic shield, at least two second metallic strips formed integrally with said first metallic strip and extending from one side of said first metallic strip and at least tow third metallic strips formed integrally with said first metallic strip and extending from the other side of said first metallic strip, each of said second and third metallic strips consisting of a first portion extending immediately from said first metallic strip and a second portion extending from said first portion, each of said second and third metallic strips being bent so that the folding lines between the main plane of said second and third metallic strips and the main plane of said first metallic strip are generally parallel to the axis of said tubular metallic shield, the directions of the bending of said second and third metallic strips being opposed to each other, one side edge of each of said first portions engaging with the respective inclined surface of said trapezoidal projection, the one side edge of each of said second portions contacting the internal surface of said casing and the other side edge of each of said second portions contacting the outer surface of said tubular metallic shield.
 7. The vacuum interrupter of claim 6, in which each of said first and second portions have an abutting portion extending from one side of each of said first and second portions, each of said abutting portions being bent so that the plane of each of said abutting portions becomes generally perpendicular to the plane of said first and second portions, and each of said abutting portions engages with either the inclined surfaces of said trapezoidal projection or the internal surface of said casing.
 8. The vacuum interrupter of claim 6, in which the first portion of each of said second and third strips is disposed at an angle to said first strip and said second portion of each of said second and third strips is disposed along a line substantially transverse to said first strip.
 9. The vacuum interrupter of claim 8, in which the first portion of said second strip extending in an opposite direction from said first strip compared to the first portion of said third strip.
 10. The vacuum interrupter of claim 9, in which said supporting member includes a pair of second metallic strips extending from one side of said first strip and a pair of third metallic strips extending from the other side of said first metallic strip, the first portions of each of said second strips being directed toward each other and the first portions of each of said third strips being directed away from each other. 